For image guided procedures using, for example, ultrasound and magnetic resonance imaging (MRI) (e.g., magnetic resonance (MR) guided high-intensity focused ultrasound (HIFU)), an MR coil of an MRI system and a transducer of an ultrasound device are in direct competition. For example, the MR coil and the ultrasound transducer are in direct competition spatially. For the MR coil, a solid angle with respect to a target volume covered by the MR coil at least partially defines a signal-to-noise (SNR) ratio of the MRI system. For the HIFU transducer, a solid angle used by the transducer defines how much power may be applied and also has an impact on the focus shape and which areas may be reached. For a diagnostic ultrasound transducer, the solid angle has an impact on the resolution and imaging coverage.
The MR coil and the ultrasound transducer are also in direct competition as a result of electro-magnetic coupling between the MR coil and the ultrasound transducer. The ultrasound transducer includes a number of conducting structures such as, for example, a ground plane, shielding, and cables. The MR coil may couple to these conducting structures, thus resulting in detuning and additional coil losses. The electrical signals from the ultrasound transducer may also couple to the MR coil, thus interfering with the sensitive MR signal.
In the prior art, the direct competition between the MR coil and the ultrasound transducer is addressed, for example, by positioning the MR coil around or behind the ultrasound transducer. For example, a large loop MR coil may be used, and an opening formed by the loop MRI coil serves as an acoustic window. This has disadvantages that the MR coil is electrically large and difficult to realize at higher field strength. Also, the large size of the MR coil places the inductive loop far from the signal emitting spins of the target volume. As another example, the MR coil is positioned behind the ultrasound transducer. This too places the MR coil further away from the target volume. Also, the MR coil may couple to the ultrasound transducer, thus reducing the quality factor of the MR coil and therefore the attainable SNR.
The direct competition between the MR coil and the ultrasound transducer may also be addressed, for example, by designing the MR coil and/or the ultrasound transducer such that coupling between the MR coil and the US transducer is minimized. For example, floating grounds and baluns, and smaller individual ultrasound transducers may be provided.